>Earthquakes associated with magmatic movement underground are in a completely different class than earthquakes, for instance, on the San Andreas Fault.Would you explain what you mean, please? See this link (click here). Wouldn't the subduction in PNG aggravate the already hot volcanoes in that area? When the Pacific plate subducts under Mt. St. Helens and Mt. Hood, it would heat up points in the cascades, therefore, how is this a different class? The whole Pacific Rim seems to be rattling like a pressure cooker and after the 7.6 and 7.7 in Fiji on August 19, volcanoes acted up in Hawaii and Japan, so it looks like there might be a relationship between the quakes and volcanoes.
Cross-section of the subduction zone below New Britain. From Johnson (1976).
Let's take the case of the earthquake swarms (hundreds of smallish quakes per day) associated with magmatic movement in the Long Valley Caldera, California area, in 1980:
"A collapsed volcano in the Eastern Sierra near Mammoth Lakes has been generating anxiety-provoking seismic swarms for decades, if not centuries, and geologists have speculated widely about the underground processes causing these quakes.
Now, a Berkeley seismologist has discovered evidence that some of these earthquakes are being caused by locked, water-saturated faults cracking open several miles underground, shoved apart by sudden increases in fluid pressure.
Based on his analysis of seismic data from quakes in the volcanic crater, Assistant Professor of Geophysics Douglas Dreger concludes that magma moving through large cracks periodically encounters fractures saturated by water. The magma heats them so suddenly that the expanding water, steam or bubbles drive the rocks apart as much as four inches in a matter of seconds.
The sudden expansion triggers an earthquake.
'As the fluid is heated, it pressurizes, reducing the frictional stress along the fault and allowing the fault to move, generating an earthquake,' said Dreger. 'As the pressurization continues, you may have continued deformation, but without seismic activity.'"
Compare the magmatic movement model above with the traditional elastic strain theory, using the 1872 Owens Valley quake +/- 8.3 as an example:
Faults and earthquakes are the result of deep-seated forces, called tectonic stress, which gradually deform the rocks of the earth's crust until rupture occurs. The rock deformation, called strain, is largely elastic and is stored in the rocks as elastic strain energy. When the strength of the rock is exceeded, rupture occurs, usually along faults which are zones of weakness. The rocks on opposite sides of the fault slide past each other as the rocks spring back to a relaxed position. The strain energy is released partly as heat and partly as elastic waves called seismic waves. The passage of these seismic waves produces the ground-shaking of an earthquake.
Since 1850 there have been three great earthquakes (M 8 or greater) in California. Both the Fort Tejon earthquake of 1857 and the famous San Francisco earthquake of 1906 resulted from movement along the San Andreas fault. The Owens Valley earthquake of 1872 was produced by movement along the Owens Valley fault on the east side of the Sierra Nevada Mountains. This great earthquake killed twenty-seven people and damaged or destroyed nearly every building in the towns of Lone Pine and Independence. This earthquake occurred at 2:30 in the morning, and Californians from Red Bluff to San Diego were awakened by the shaking of their beds. The quake produced about 4 feet vertical displacement and up to 18 feet lateral displacement.
When I said "different class" of earthquake, I was referring to the causal mechanisms, rather than the magnitude or intensity of shaking. A third class of seismic event includes meteorite impacts. I hope this helps.